Methanol is primarily used to produce formaldehyde, methyl tertiary butyl ether (MTBE) and acetic acid, with smaller amounts going into the manufacture of dimethyl terephthalate (DMT), methylmethacrylate (MMA), chloromethanes, methylamines, glycol methyl ethers, and fuels. It also has many general solvent and antifreeze uses, such as being a component for paint strippers, car windshield washer compounds and a de-icer for natural gas pipelines
A major use of methyl acetate is as a low toxicity solvent in glues, paints and a broad range of coating and ink resin applications. Methyl acetate also finds use as a feedstock in the production of acetic anhydride.
Methanol may be produced on a commercial basis by the conversion of synthesis gas containing carbon monoxide, hydrogen and optionally carbon dioxide over a suitable catalyst according to the overall reaction:2H2+CO⇄CH3OH
Widely used catalysts for methanol synthesis from synthesis gas are based on copper.
WO 03/097523 describes a plant and process that produces methanol and acetic acid under substantially stoichiometric conditions, wherein an unadjusted syngas having an R ratio less than 2.0 is provided. All or part of the unadjusted syngas is supplied to a separator unit to recover CO2, CO and hydrogen. At least a portion of any one or combination of the recovered CO2, CO and hydrogen is added to any remaining syngas not so treated or alternatively combines in the absence of any remaining unadjusted syngas to yield an adjusted syngas with an R ratio of 2.0 to 2.9 which is used to produce methanol. Any recovered CO2 not used to adjust the R ratio of the unadjusted syngas can be supplied to the reformer to enhance CO production. At least a portion of the recovered CO is reacted in the acetic acid reactor with at least a portion of the produced methanol to produce acetic acid or an acetic acid precursor by a conventional process.
Methyl acetate may be produced by an integrated process as described in EP-A-0529868, in which process methanol and acetic acid are reacted in an esterification reactor and the methyl acetate is recovered by distillation and the water by azeotropic distillation, the process is operated in ‘standby’ mode by shutting off the methanol and acetic acid feds to the esterification reactor and recycling the methyl acetate and water to the esterification reactor so that the process may be rapidly restarted.
Methyl acetate may be produced, as described, for example in WO 2006/121778, by carbonylating dimethyl ether with carbon monoxide in the presence of a zeolite carbonylation catalyst, such as a mordenite zeolite.
The production of methyl acetate by the carbonylation of dimethyl ether may also be carried out using mixtures of carbon monoxide and hydrogen, as described, for example in WO 2008/132438. According to WO 2008/132438, the molar ratio of carbon monoxide:hydrogen for use in the carbonylation step may be in the range 1:3 to 15:1, such as 1:1 to 10:1, for example, 1:1 to 4:1.
WO 01/07393 describes a process for the catalytic conversion of a feedstock comprising carbon monoxide and hydrogen to produce at least one of an alcohol, ether and mixtures thereof and reacting carbon monoxide with the at least one of an alcohol, ether and mixtures thereof in the presence of a catalyst selected from solid super acids, heteropolyacids, clays, zeolites and molecular sieves, in the absence of a halide promoter, under conditions of temperature and pressure sufficient to produce at least one of an ester, acid, acid anhydride and mixtures thereof.
GB 1306863 describes a process for producing acetic acid, which comprises the following steps: (a) reacting a gaseous mixture of carbon monoxide and hydrogen in a molar ratio of 1:not more than 0.5, with methanol in the gas phase in the presence of a transition metal catalyst and a halogen-containing compound co-catalyst until no more than half of the carbon monoxide is consumed; (b) cooling the reacted gas obtained in step (a), separating the cooled gas into a liquid component containing acetic acid and a gaseous component containing unreacted carbon monoxide and hydrogen, and withdrawing the acetic acid from the reaction system; (c) washing the gaseous component from step (b) with cold methanol; and (d) reacting the washed gaseous component from step (c) in the presence of a copper-containing catalyst to yield methanol and passing this methanol to step (a).
U.S. Pat. No. 5,286,900 relates to a process for preparing an acetic acid product selected from acetic acid, methyl acetate, acetic anhydride and mixtures thereof by conversion of a synthesis gas comprising hydrogen and carbon oxides, said process comprising the steps of: (i) introducing synthesis gas into a first reactor at a pressure of 5-200 bar and a temperature of 150-400° C., and catalytically converting the synthesis gas into methanol and dimethyl ether and (ii) carbonylating the methanol and dimethyl ether formed in step (i) by passing the entire effluent from the first reactor to a second reactor and carbonylating therein, at a pressure of 1-800 bar and a temperature of 100-500° C. in the presence of a catalyst, the methanol and dimethyl ether to an acetic acid product.
EP-A-0801050 describes a process for the preparation of acetic acid which comprises catalytic steps of converting hydrogen and carbon monoxide in the synthesis gas to a mixed process stream containing methanol and dimethyl ether and carbonylating methanol and dimethyl ether formed in the process stream into acetic acid.
U.S. Pat. No. 5,502,243 describes a process wherein oxygenated acetyl compounds ethylidene acetate, acetic acid, acetic anhydride, acetaldehyde and methyl acetate are produced directly from synthesis gas and dimethyl ether in a catalyzed liquid phase reaction system. The inclusion of carbon dioxide in the synthesis gas in selected amounts increases the overall yield of oxygenated acetyl compounds from the reactant dimethyl ether. When methanol is included in the reactor feed, the addition of carbon dioxide significantly improves the molar selectivity to ethylidene diacetate.
EP-A-0566370 describes a process for the production of ethylidene diacetate, acetic acid, acetic anhydride and methyl acetate directly from synthesis gas via an intermediate product stream containing dimethyl ether. Dimethyl ether is produced from synthesis gas in a first liquid phase reactor and the reactor effluent comprising dimethyl ether, methanol and unreacted synthesis gas flows to a second liquid phase reactor containing acetic acid in which the oxygenated acetyl compounds are synthesized catalytically. Vinyl acetate and additional acetic acid optionally are produced by pyrolysis of ethylidene diacetate in a separate reactor system. Synthesis gas is preferably obtained by partial oxidation of a hydrocarbon feedstock such as natural gas. Optionally a portion of the acetic acid co-product is recycled to the partial oxidation reactor for conversion into additional synthesis gas.
Synthesis gas comprises carbon monoxide and hydrogen. Optionally carbon dioxide is included. The synthesis gas ratio or stoichiometric number (SN) of a synthesis gas composition is conventionally calculated asSN═(H2—CO2)/(CO+CO2)wherein H2, CO and CO2 represent the composition of the gas on a molar basis.
Desirably, the optimum stoichiometric number of a synthesis gas for use in methanol production is 2.05. Typically, however, processes for the production of methyl acetate by the carbonylation of dimethyl ether with synthesis gas employ synthesis gas with a stoichiometric excess of carbon monoxide. Thus a major drawback in integrated carbonylation and methanol synthesis processes is that the hydrogen:carbon monoxide ratios desirable for methanol synthesis are significantly higher than the desired ratios for carbonylation.
A further drawback of processes for the carbonylation of dimethyl ether is that a purge gas must be removed from the process to prevent recycle components from reaching unacceptable levels in the reactor. Typically, purge gases are disposed of by burning. Purge gas from the carbonylation process contains carbon monoxide and invariably contains some dimethyl ether and methyl acetate. Therefore, the removal of these components by purging represents a loss of values and reduces the overall efficiency of the process.
As described above, processes for the carbonylation of dimethyl ether with synthesis gas typically employ synthesis gas with a stoichiometric excess of carbon monoxide. This results in unconsumed carbon monoxide being withdrawn (together with hydrogen which generally remains unconsumed in the process) from the process as part of the carbonylation product stream. Typically, to avoid loss of carbon monoxide from the process it is recycled together with the unconsumed hydrogen to the carbonylation reactor. A disadvantage of this is that hydrogen builds-up in the reactor and an undesirable reduction in the carbonylation reaction rate is observed.
A yet further drawback is that the introduction of synthesis gas streams containing methyl acetate to methanol synthesis processes has now been found to result in undesirable side-reactions and/or by-products, such as ethanol and acetic acid resulting in a detrimental loss of catalytic performance and/or methanol productivity.